Voltage level shifting circuits are well known in the art. Level shifting circuits are used when interfacing different types of circuits to each other, such as when interfacing circuits operating at one particular voltage to circuits operating at another voltage level, for example when interfacing emitter coupled logic operating at a few hundred millivolts (ECL) to complementary metal oxide semiconductor (CMOS) circuits operating at several volts potential. Analog integrated circuits which operate on low single power supply voltage levels (e.g. Vcc=2.7 volts) typically use the half supply voltage (i.e., Vcc/2) as the midrange direct current (DC) reference for analog signal processing functions such as filters or amplifiers. The half supply voltage may not, however, be the reference level of choice for all signal processing functions within the same integrated circuit (IC). A problem of not getting enough signal swing when operating at low supply voltages (such as 1.8 V or 2.7 V) becomes apparent in the following example.
Referring to FIG. 1, an example of a prior art electronic CMOS circuit 100 using a typical N-channel metal oxide semiconductor-field effect transistor (NMOS-FET) differential pair circuit 102, 104 is shown. The differential pair (102, 104) is biased from voltage sources In (+) and In(-) each preferably equal to 0.9 volts DC for a supply voltage Vcc of preferably 1.8 volts. Assuming a gate-to source forward bias potential of 0.8 volts, the available negative dynamic range (i.e., AC amplitude) is only 100 millivolts (mV), whereas the available positive dynamic range is nearly Vcc/2 or 0.9 volts. The 100 mV negative dynamic range would cause clipping of an incoming AC signal having an amplitude greater than 100 mV. The maximum desired theoretical swing for the circuit 100 is, .+-.(100 mV+900 mV)/2=.+-.500 mV. Thus, the optimum bias level for this particular circuit element is (Vcc/2)+0.4 volts or 1.3 volts DC. This yields a symmetrical AC dynamic range of .+-.500 millivolts (mV) which is a five-fold increase over the original 100 mV. A circuit that would provide precision DC level shifting (in this case +400 mV) would improve the dynamic signal swing of the circuit 100.
Other analog signal processing functions within the same IC may require a different optimum DC reference level than Vcc/2. Achieving the maximum dynamic range in an analog circuit is a primary design objective as this is equivalent to achieving a maximum signal-to-noise ratio performance. In a radio receiver, for example, achieving a maximum signal-to noise ratio permits the radio to either operate over greater distances or lower the required transmit power. These are fundamental system design goals. Thus, there is a need for a circuit that provides a precision DC level shift at the input of an analog element and a corresponding complementary level shift at the output which would permit all of the analog circuit functions to operate with a maximum dynamic range.